Under the plan, drivers would pay for using every single road in the country. Sophisticated satellite and global positioning equipment would track vehicles, charging them according to the route they take.

Busy roads at busy times of day could cost as much as $2 per mile, according to preliminary proposals. Small rural routes would cost just a few cents a mile.

The idea, set to be formally unveiled Thursday in a speech by Transport Minister Alastair Darling, would be unprecedented. Several countries have talked about road pricing, or variable pricing, for decades, but nothing on this scale has ever been considered before.

And despite some grumbling that satellite tracking would breach privacy, it's an idea that may be spreading.

A pilot project similar to the British plan began near Seattle late last year, but results won't be reported until 2006. Cities from New York to Moscow, meanwhile, have been watching London's landmark congestion charge scheme with interest.

"It's a national issue now," says a government spokeswoman. "As the country gets more prosperous and people use their cars more, it's an issue we'll have to address."

A government study found that road pricing could reduce congestion by 40 percent and result in $20 billion in time savings, she adds.

A fully functioning nationwide system - involving roadside devices that monitor every journey of every vehicle - is still a decade away. But the government hopes to legislate in the next couple of years.

The impetus is a simple case of supply and demand. Demand for roads has risen dramatically as vehicle numbers have increased more than tenfold in the past 50 years, to 28 million. Yet supply, in the form of road capacity, has increased by only 20 percent, according to Philip Hale of the RAC motorists' organization. The only way to smooth out the imbalance is to increase the supply of roads or introduce a pricing element to deplete demand.

"If we were to price for other goods the way we do for road space, there would be huge inefficiencies and massive excess demand," says Professor David Begg, a former government adviser on transport and the director of the center for transport policy. "We are pricing for road use the way the former Soviet Union used to ration bread - through queues, not prices."

"We can't increase much more because of the island we are," says Mr. Hale. "You can fit Britain inside Minnesota. We can't build our way out of this. It's environmentally and politically unacceptable. But we can manage the road traffic we have."

London has discovered just that. Two years ago, it introduced a flat tax of £5 ($9) for driving into the city center. Today, it reports congestion reduced by 30 percent, traffic down by 18 percent, and hundreds of millions of pounds raised to improve public transport.

Indeed, a three-mile journey across the zone could take more than an hour before the new charge was introduced in February 2003. Now it can take as little as 20 minutes. Traffic is notably thicker on weekends, when the charge is not levied.

"We're convinced it's been a success," says Alun Shurmer of the Transport for London authority. But he adds that the London scheme could not be implemented nationally. It works via a network of 700 cameras that record license plate numbers and check them against database lists of those who have already paid.

The national scheme would have to work very differently. To survey that number of roads would require a silly number of cameras. Instead, every vehicle would have a black box to allow a satellite system to track their journey. Tolls would be automatically calculated, possibly deducted from prepaid accounts.

The new tolls would partly replace existing motoring taxes - a flat tax on car ownership and steep duties levied on gasoline. The upshot would be that occasional motorists might end up paying less to run their cars. Heavy road users would, of course, pay more. The RAC estimates that it costs $9,000 annually to keep, run, and maintain a car in Britain.

Professor Stephen Glaister, an expert in transport policy at Imperial College London and a firm advocate of road pricing, says the cost of the technology may prove prohibitive. "It may turn out that it isn't worth it in anywhere other than most congested places," he says.

But he is convinced that something will have to be done about Britain's congestion time bomb, and adds that the London scheme proves that people do respond to price "incentives."

"Demand for car ownership and use have grown out in proportion to real income, as it has done in the US; but for all sorts of reasons, we haven't invested in road capacity to keep up," he says. "The densities are very much higher here than anywhere else in the developed world, which means congestion will hit us sooner than it will elsewhere."